The great silence: Just 4 in 10,000 galaxies may host intelligent aliens
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Alien life capable of communicating across interstellar space might not be able to evolve if its home planet doesn't possess plate tectonics, not to mention just the right amount of water and dry land.
Plate tectonics are absolutely essential if complex life is to evolve, argue Robert Stern of the University of Texas at Dallas and Taras Gerya of ETH Zurich in Switzerland. On Earth, complex multicellular life appeared during a period known as the Cambrian explosion, 539 million years ago.
"We believe that the onset of modern-day-style plate tectonics greatly accelerated the evolution of complex life and was one of the major causes of the Cambrian explosion," Gerya told Space.com.
Plate tectonics describes the process of continental plates, which are buoyed up on a molten mantle, sliding over one another, leading to subduction zones and mountains, rift valleys and volcanoes, as well as earthquakes.
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The modern-day form of plate tectonics, say Stern and Gerya, only began between a billion and half a billion years ago, in a geological era known as the Neoproterozoic. Prior to that, Earth had what's known as stagnant lid tectonics: Earth's crust, called the lithosphere, was one solid piece and wasn't broken into different plates. The change to modern-day plate tectonics only happened once the lithosphere had cooled enough to grow sufficiently dense and strong to be capable of being subducted — that is, to be pushed under other parts of the lithosphere for a significant amount of time before being recycled back onto the surface where two tectonic plates are moving apart.
The environmental stresses that modern-day plate tectonics places on the biosphere could have instigated the evolution of complex life a little over half a billion years ago, as life suddenly found itself living in an environment where it was forced to adapt or die, creating an evolutionary pressure that pushed the development of all manner of life that existed in the oceans and on the dry land associated with the continental plates. Given that kickstart, life eventually — through no design or evolutionary imperative other than natural selection — ended up evolving into us, the idea goes.
"The long-lasting coexistence of oceans with dry land seems critical for obtaining intelligent life and technological civilizations as the result of biological evolution," said Gerya. "But having continents and oceans is not sufficient on their own, because life's evolution is very slow. In order to accelerate it, plate tectonics is needed."
However, there's a problem. Earth is the only planet in the solar system to have plate tectonics. What's more, models indicate that plate tectonics could be rare, especially on a class of exoplanets known as super-Earths, where the stagnant lid configuration could dominate.
Coupled with the need for plate tectonics is the need for oceans and continents. Models of planetary formation indicate that planets covered entirely in oceans dozens of miles deep could be common, as could desert worlds with no water at all. Earth, with its relatively thin veneer of ocean water and topography that allows continents to rise above the oceans, seems to occupy a sweet spot that is carefully balanced between the two extremes of deep ocean planets and dry desert worlds.
Having oceans is crucial because it is strongly suspected that life on Earth began in the sea. Land is also critical, not only for providing nutrients via weathering and facilitating the carbon cycle, but also for enabling combustion (in concert with oxygen) that can lead to technology when harnessed by intelligent life.
If planets with plate tectonics, as well as the right amount of water and land, are rare, then technological, communicative, alien life may also be rare.
"What we have tried to explain is, why have we not been contacted?" said Gerya.
Related: Fermi Paradox: Where are the aliens?
To illustrate this, Gerya and Stern used the Drake equation. Devised in 1961 by the late SETI pioneer Frank Drake, it was intended to provide an agenda for the first-ever SETI (search for extraterrestrial intelligence) scientific conference, held in that year at the Green Bank Observatory in West Virginia, by summarizing the various factors required for the development of technological civilizations, resulting in an estimate of the number of extraterrestrial civilizations that might exist. However, it should be noted that the Drake equation is more of a thought experiment to highlight what we know and what we don't know about the evolution of technological life, rather than an absolute guide to the number of civilizations out there.
"Previous estimates for the lower limit of the number of civilizations in our galaxy were rather high," said Gerya.
One of the terms of the Drake equation is fi, the fraction of exoplanets that develop intelligent life (how we define "intelligence" in this context is still debated, but the modern way of thinking includes all intelligent animals, such as chimps and dolphins). Stern and Gerya argue that fi should be the product of two more terms, specifically the fraction of planets with both continents and oceans (foc), and the fraction of planets with long-lasting plate tectonics (fpt).
However, given the apparent rarity of plate tectonics, and worlds that can have oceans and continents, Stern and Gerya find that fi is a very small number. They estimate that just 17% of exoplanets have plate tectonics, and the proportion with just the right amount of water and land is likely even smaller — between 0.02% and 1%. Multiply these together and they give a value of fi as between 0.003% and 0.2%.
Then, by plugging this value into the Drake equation, Stern and Gerya arrive at a value for the number of extraterrestrial civilizations as somewhere between 0.0004 and 20,000. That's still quite a large range, the result of the other terms in the Drake equation not being known well, if at all. However, it is still orders of magnitude less than the value of a million civilizations that Drake predicted in the 1960s.
"A value of 0.0004 means that there could be as few as 4 civilizations per 10,000 galaxies," said Taras.
There are several caveats to all this. One is, as mentioned, that some of the other terms of the Drake equation such as the fraction of planets that evolve life in the first place, the fraction with intelligent life that develops technology and the lifetime of those civilizations are completely unknown. If their values turn out to be extremely high — for example, if civilizations typically survive for billions of years — then the chances of more of them being around now will increase.
Another caveat is that while, in general, life as we know it needs plate tectonics, oceans and land to evolve and thrive, it is possible to imagine scenarios where technological, ocean-dwelling life that never steps foot on land could evolve. However, these would be specific cases, outliers that are the exception to the rule.
There's also a risk of jumping the gun when saying that we haven't been contacted yet. SETI astronomer Jill Tarter is fond of saying that if, the galaxy were an ocean, we'd have searched only a cup's worth of it. While the search has accelerated recently thanks to the ambitious Breakthrough Listen project, the point still stands. We've not searched every star yet, and those that we have searched, we have not listened to or watched for very long. We could easily have missed an extraterrestrial signal.
A final point to consider is that of the "Great Filter." This is a concept first proposed by the economist and futurist Robin Hanson, which suggests that there might be some universal bottleneck in the evolution of all life that prevents technological civilizations from existing. In Stern and Gerya's model, that bottleneck is provided by the lack of plate tectonics, oceans and continents. However, despite their estimate for the number of civilizations being low, it is non-zero, and there is a school of thought that plays into the Copernican principle, which states that Earth should not be treated as special and is just another planet orbiting a humdrum star. Therefore, if life can evolve on Earth, it should be able to evolve on many planets, because Earth shouldn't be special. The question then becomes, At what point does the Great Filter kick in?
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Perhaps Stern and Gerya have jumped the gun, declaring that planets with plate tectonics and just the right amount of water and land are rare, before we have the observational evidence to support that statement.
"Of course, it would be ideal to have observational data on how common continents, oceans and plate tectonics are on exoplanets," said Gerya. "Unfortunately, this is far beyond our current observation capacities. On the other hand, the planetary formation process is to some extent understood, and planetary formation models are capable of delivering predictions about what we can expect. Those predictions can be used to evaluate the probability of rocky exoplanets having continents, oceans and plate tectonics."
If Stern and Gerya are correct, then we could very well be effectively alone in the universe. If that's the case, we have an enormous responsibility to shoulder. "We should take all possible care to preserve our own — very rare! — civilization," said Gerya. Otherwise, we could kill ourselves off and render extinct the only technological life in our Milky Way galaxy.
Stern and Gerya's analysis was published on April 12 in the journal Scientific Reports.
